20.11 Quitting

Typing C-g while a Lisp function is running causes Emacs to
quit whatever it is doing. This means that control returns to the
innermost active command loop.

Typing C-g while the command loop is waiting for keyboard input
does not cause a quit; it acts as an ordinary input character. In the
simplest case, you cannot tell the difference, because C-g
normally runs the command keyboard-quit, whose effect is to quit.
However, when C-g follows a prefix key, they combine to form an
undefined key. The effect is to cancel the prefix key as well as any
prefix argument.

In the minibuffer, C-g has a different definition: it aborts out
of the minibuffer. This means, in effect, that it exits the minibuffer
and then quits. (Simply quitting would return to the command loop
within the minibuffer.) The reason why C-g does not quit
directly when the command reader is reading input is so that its meaning
can be redefined in the minibuffer in this way. C-g following a
prefix key is not redefined in the minibuffer, and it has its normal
effect of canceling the prefix key and prefix argument. This too
would not be possible if C-g always quit directly.

When C-g does directly quit, it does so by setting the variable
quit-flag to t. Emacs checks this variable at appropriate
times and quits if it is not nil. Setting quit-flag
non-nil in any way thus causes a quit.

At the level of C code, quitting cannot happen just anywhere; only at the
special places that check quit-flag. The reason for this is
that quitting at other places might leave an inconsistency in Emacs's
internal state. Because quitting is delayed until a safe place, quitting
cannot make Emacs crash.

Certain functions such as read-key-sequence or
read-quoted-char prevent quitting entirely even though they wait
for input. Instead of quitting, C-g serves as the requested
input. In the case of read-key-sequence, this serves to bring
about the special behavior of C-g in the command loop. In the
case of read-quoted-char, this is so that C-q can be used
to quote a C-g.

You can prevent quitting for a portion of a Lisp function by binding
the variable inhibit-quit to a non-nil value. Then,
although C-g still sets quit-flag to t as usual, the
usual result of this—a quit—is prevented. Eventually,
inhibit-quit will become nil again, such as when its
binding is unwound at the end of a let form. At that time, if
quit-flag is still non-nil, the requested quit happens
immediately. This behavior is ideal when you wish to make sure that
quitting does not happen within a “critical section” of the program.

In some functions (such as read-quoted-char), C-g is
handled in a special way that does not involve quitting. This is done
by reading the input with inhibit-quit bound to t, and
setting quit-flag to nil before inhibit-quit
becomes nil again. This excerpt from the definition of
read-quoted-char shows how this is done; it also shows that
normal quitting is permitted after the first character of input.

This variable determines whether Emacs should quit when quit-flag
is set to a value other than nil. If inhibit-quit is
non-nil, then quit-flag has no special effect.

— Macro: with-local-quit body...

This macro executes body forms in sequence, but allows quitting, at
least locally, within body even if inhibit-quit was
non-nil outside this construct. It returns the value of the
last form in body, unless exited by quitting, in which case
it returns nil.

If inhibit-quit is nil on entry to with-local-quit,
it only executes the body, and setting quit-flag causes
a normal quit. However, if inhibit-quit is non-nil so
that ordinary quitting is delayed, a non-nilquit-flag
triggers a special kind of local quit. This ends the execution of
body and exits the with-local-quit body with
quit-flag still non-nil, so that another (ordinary) quit
will happen as soon as that is allowed. If quit-flag is
already non-nil at the beginning of body, the local quit
happens immediately and the body doesn't execute at all.

This macro is mainly useful in functions that can be called from
timers, process filters, process sentinels, pre-command-hook,
post-command-hook, and other places where inhibit-quit is
normally bound to t.

— Command: keyboard-quit

This function signals the quit condition with (signal 'quit
nil). This is the same thing that quitting does. (See signal
in Errors.)

You can specify a character other than C-g to use for quitting.
See the function set-input-mode in Input Modes.